its a measure of the efficiency of a structure - normally associated with things like bridges.

It compares the weight of the structure itself to the amount of weight it can carry/support without collapsing.

For example the miniature wood-stick bridges you build at school will have a very high strength to weight ratio - in the hundreds, sometimes the thousands. This is because they weigh only a couple dozen grams, yet are able in some instances to hold one or two hundred kilograms without breaking!

A modern-day metal bridge however will have a strength to weight ratio nearer to 1.